Comparison of Target Delineation Methods in Dynamic Tumor Tracking Radiation Therapy for Lung Tumors Based on Internal Fiducial Markers

Abstract

We previously defined a new target delineation method for dynamic tumor tracking (DTT) radiotherapy (RT) based on internal fiducial markers which were inserted near a tumor. An internal target volume for tracking (ITVtracking) is assumed to compensate for the positional variations between the tumor and fiducial makers and tumor deformation. In general, a three-dimension (3D) gross tumor volume (GTV) to planning target volume isotopic margin of 5 mm is widely used in DTT RT with a linear accelerator with a robotic arm. However, thus far, there is no theoretical or quantitative evaluation and comparison between the “ITVtracking method” and the “isotropic margin method.” The purpose of this study was to compare between these two methods. We evaluated the four-dimensional (4D) computed tomography (CT) images of patients who received DTT RT at our institution. Ten phases of 4DCT were acquired with a 16-row multidetector CT during free breathing. The GTV in each phase was delineated semiautomatically with manual modifications. ITVtracking was created as a composite of the GTVs in 10-phase images of 4DCT sets, which were transposed to the centroid of their markers coincided at a reference CT phase. The reference CT phase was selected based on image quality and used for dose calculation. ITVs with the isotropic margin method (ITVisos) were created by adding a 5-mm margin to the GTV at the same reference CT phase. We compared the values of ITVtracking and ITViso through statistical evaluation with a paired t-test, and we evaluated their overlapping volume and Dice’s similarity coefficient (DSC). Moreover, we determined the indispensable margin for the GTV at the reference CT phase to encompass the entire ITVtracking. Between September 2011 and December 2016, 26 4DCT sets in 28 patients were evaluated. Two CT sets could not be evaluated because the 4DCT images were not reconstructed. The median tumor diameter was 21 mm (range, 10–44 mm). The end-exhale phase was used as the reference CT phase in 17 patients, and the mid-ventilation phase was used in 9 patients. The median values of ITViso and ITVtracking were 18.5 and 12.4 mL, respectively (range, 5.1–70.3 and 1.8–64.8 mL, p < 0.01). The ITViso included the entire ITVtracking in nine patients. The average rate of non-inclusion of ITVtracking in ITViso was 5.7% (range, 0.1–31.8%) in the other 17 patients. The median DSC was 0.73 (range, 0.46–0.89). The median margin for the GTV to encompass the entire ITVtracking was 6.5 mm (range, 2–15 mm). ITViso was statistically larger than ITVtracking, but ITViso did not always include ITVtracking. The ITVtracking method enables a reasonable reduction of the target size.